Calcitonin gene-related peptide potentiates nicotinic acetylcholine receptor-operated slow Ca super(2+) mobilization at mouse muscle endplates

The involvement of calcitonin gene-related peptide (CGRP) in the non-contractile slow Ca super(2+) mobilization induced by prolonged nicotinic stimulation was investigated by measurement of [Ca super(2+)] sub(i) levels in mouse single muscle cells (flexor digitorum brevis; FDB) loaded with a Ca super(2+) indicator fluo-3 using confocal laser scanning microscopy. CGRP (3-30 nM) potentiated acetylcholine (ACh, 1 mu M)-elicited slow Ca super(2+) mobilization in a concentration-dependent manner. The potentiation by CGRP of the slow Ca super(2+) component was greatly depressed by a competitive nicotinic antagonist (+)-tubocurarine (5 mu M). The Ca super(2+) channel blocker nitrendipine (1 mu M) affected neither ACh responses nor the CGRP potentiation. The slow Ca super(2+) component was completely abolished by reducing [Ca super(2+)] sub(0) from 2.5 to 0.25 mM whereas the fast component was not affected. The CGRP-induced potentiation of slow Ca super(2+) signal was also depressed by decreasing [Ca super(2+)] sub(0). Isoproterenol (30 mu M) and 8-bromo-adenosine 3 theta ,5 theta -cyclic monophosphate (1 mM) potentiated the ACh-elicited slow Ca super(2+) response. The potentiation by CGRP of the slow Ca super(2+) component was completely abolished by a protein kinase-A inhibitor H-89 (1 mu M). These findings indicate that CGRP potentiates the nicotinic ACh receptor-operated slow Ca super(2+) signal via the activation of protein kinase-A system at the skeletal muscle endplates.